In recent years, in a cellular mobile communication system, a technique for realizing high transmission rate by using high frequency wireless communication bands has been vigorously examined in order to realize transmission of mass data with the advance of multimedia information. In the cellular mobile communication system, there are, for example, a Long Term Evolution (LTE) scheme, a Wideband Code Division Multiple Access (W-COMA) scheme or a cdma2000 scheme which is classified as 3rd Generation (3G), a Global System for Mobile Communication (GSM) (registered trademark) scheme which is classified as 2nd Generation (2G), and the like, and a plurality of mobile communication systems are mixed. In a case where a single wireless communication mobile terminal (hereinafter, simply referred to as “terminal”) can be connected to a plurality of different mobile communication systems, a different-type network handover which is a handover between different types of mobile communication systems may occur. The different-type network handover is a handover or the like, for example, from an LTE network in which communication can be performed at a high transmission rate to a 3G network in which communication is performed at a transmission rate lower than that in the LTE network, and is called an Inter-RAT (Radio Access Technology) handover. In addition, an Intra-RAT handover refers to an identical-type network handover such as a handover, for example, from an LTE network to an LTE network.
FIG. 19 is a timing chart illustrating procedures when an Intra-RAT handover and an Inter-RAT handover occur. As illustrated in FIG. 19, a terminal receives a reception quality measurement configuration (Measurement Configuration) from a currently connected base station (hereinafter, referred to as a “serving base station”; however, also referred to as a “source base station” when the base station is a handover source). The reception quality measurement configuration indicates a condition for reporting a reception quality measurement result to the serving base station, content to be reported, a frequency of reports, or the like. The terminal measures reception quality of a serving cell and a neighbor cell while communicating with the serving base station. If a condition designated in the reception quality measurement configuration is satisfied, the terminal transmits a report (Measurement Report) regarding the reception quality measurement result to the serving base station.
When it is determined that a handover is performed based on the report (Measurement Report) sent from the terminal, the serving base station selects a base station (target base station) which is a handover destination, and performs a handover procedure with the target base station. Next, the serving base station which is a source base station sends a handover instruction to the terminal. The terminal performs a handover procedure with a base station (target base station) designated by the handover instruction in response to the handover instruction sent from the serving base station. When the handover procedure is completed, the terminal starts communication with the target base station.